This invention relates to a method of forming plastic tubing made by a blow head, which method provides internal cooling air and an exchange of the internal cooling air through a central opening in the blow head. The exterior air ring or rings mounted circumferentially around the tubing as extruded from the blow head deliver warmer air around the exterior surfaces of the tubing to establish a thermal differential for a sufficient time interval between interior and exterior surfaces of the newly-formed tubing during tube formation.
The subject method is particularly important in forming foam-film composite laminates such as from similar or dissimilar thermoplastic materials.
In the manufacture of tubing from synthetic thermoplastic material, it is well known that tubing can be cooled much more advantageously with air than with water. While water cooling in some instances is more effective, its effects are so abrupt as to result in tubing having variable wall thickness and undesirable properties. The main disadvantage of air cooling resides in its low cooling capacity. In the earlier forms of cooling apparatus of the type defined above, the operating methods required that the tubing be taken away from the blow head at relatively slow speed so that only relatively low cooling capacity has been required. In some forms of apparatus, it was not possible to increase the cooling capacity by circulation of the cooling air at a higher rate because such increase in cooling capacity by increasing the air delivery rates resulted in certain disadvantages. These included constriction of the neck of the tube by the air jet effect, or uncontrollable fluttering or vibration of the tubing when still in a plastic state, resulting in non-uniform wall thicknesses.
Increasing the cooling capacity by combining the external cooling of the tubing with simultaneous internal cooling has been achieved by air coolers which are inserted into the blown tubing and are provided with a blower for circulating the enclosed air within the tube to increase the cooling capacity; however, such methods have resulted in the condensation of evaporated constituents of certain molten materials. Various forms of apparatus have been disclosed which provide internal cooling of the tubing with a surge of air through a large central opening of a laterally-fed blow head. Various types of apparatus to achieve internal cooling has been disclosed in U.S. Pat. Nos. 3,898,028, 3,930,768, 3,966,377, 4,019,843, and 3,709,290.